This invention is directed generally to copy sheet sensing in a copier/printer, and more particularly, to an improved scheme for use in such copiers/printers in sensing when a duplexed, unfused copy sheet enters a fuser nip.
Traditionally, In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image on a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be either fixed directly upon the photosensitive member or transferred from the member to a sheet of plain paper with subsequent affixing of the image thereto.
In order to permanently affix or fuse an electroscopic toner material onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to be absorbed to some extent into the fibers of the support member which in many instances constitutes plain paper. Thereafter, as the toner material is cooled, solidification of the toner material occurs causing the toner material to be firmly bonded to the support member.
In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well-known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed rollers, at least one of which is either externally or internally heated. In this type of arrangement, the toner image contacts the surface of the heated roller member in the nip between rollers to thereby produce heating of the toner image within the nip.
In apparatus utilizing a fuser roll pair as described above, it is important that the heated roll surface be maintained within a suitable range to properly fuse the toner image to its paper support sheet. The foregoing is accomplished in a conventional manner by the employment of a temperature sensitive resistance device commonly referred to as a thermistor which is placed in physical contact with the heated roll.
In a conventional roll fuser, the fuser roll or heated member is almost always provided with a release agent applied to the surface thereof in order to prevent offsetting of toner material to the fuser roll. The release agent is, at least at the time that the application to the fuser roll has been accomplished, in the form of a liquid and has the effect of minimizing the thermal energy generated by frictional forces due to rubbing between the fuser roll surface and the thermistor. Alternatively, as shown in U.S. Pat. No. 3,849,628, the fuser roll surface could comprises an elastomeric or other highly susceptible heat generating material that does not have a release agent applied to the surface thereof.
A problem arises when it becomes necessary to sense the instant a copy sheet enters the fuser nip because it becomes extremely difficult, if not impossible, to position the embodiment of a sensing device exactly at the roll nip contact point. Sensing of the fuser roll nip contact point is important in copiers/printers when single pass duplex is practiced, i.e., when unfused images are placed onto both sides of a copy sheet and then the copy sheet is fused. It has been found that conventional fuser roll speeds for copy sheets with only one image thereon are inadequate for unfused, duplex imaged copy sheets because the images are not fused properly due to the speed of the fuser roll nip being too fast.